The present invention relates to data packet communications, and in particular, to controlling switching between communication channels of different types in a Wideband Code Division Multiple Access (W-CDMA) cellular communications network.
In current and future mobile radio communications systems, a variety of different services either are or will be provided. While mobile radio systems have traditionally provided circuit-switched services, e.g., to support voice calls, packet-switched data services are becoming increasingly utilized. Exemplary packet data services include e-mail, file transfers, and information retrieval using the Internet. Because packet data services often utilize system resources in a manner that varies over the course of a data packet session, the flow of packets is often characterized as xe2x80x9cbursty.xe2x80x9d Transmitted packet bursts are interspersed with periods where no packets are transmitted so that the xe2x80x9cdensityxe2x80x9d of packets is high for short time periods and often very low for long periods.
It is often desired that mobile communications systems be capable of accommodating both circuit-switched and packet-switched services. It is also typically desired that the limited radio bandwidth be efficiently used. Consequently, different types of radio channels may be employed to more efficiently accommodate different types of traffic to be transported across the radio interface (e.g., the radio interface between cell phones/pagers and corresponding base station(s)).
The Global System for Mobile communications (GSM) is one example of a mobile communications system that offers circuit-switched services via a Mobile Switching Center (MSC) node and packet-switched services via a General Packet Radio Service (GPRS) node. For circuit-switched guaranteed service, dedicated traffic channels are typically employed. A radio channel is dedicated to a particular mobile user and delivers frames of information as received without substantial delay, and typically provides high data throughput. For packet-switched, best effort service, common channels may be employed where plural mobile users share a common channel at the same time. Typically, a common channel delivers packets of information at a relatively low data throughput as compared to a dedicated channel. Thus, when the Quality of Service (QoS) parameter(s) requested is/are relatively high (e.g., for speech or synchronized communication, soft handover, etc.), a dedicated circuit-switched channel may be well suited to handle this kind of traffic. When the quality of service requested is relatively low (e.g., for an e-mail message, or if the user only has a small amount of data to transmit), a common, packet-switched channel may be well suited to handle this kind of traffic. Unfortunately, there is no xe2x80x9cswitchingxe2x80x9d between different types of channels in GSM/GPRS. All dedicated traffic is GSM circuit-switched, and all common traffic is GPRS packet-switched.
The selection of the appropriate channel type and channel type switching are desirable features to be included in third generation mobile systems that employ Wideband Code Division Multiple Access (W-CDMA). W-CDMA systems may support a variety of circuit-switched and packet-switched services over a wide range of bit rates, e.g., kilobits per second to megabits per second. Two radio resources in wideband CDMA used to support such services are channelization codes and transmission power. Channelization codes are used to reduce interference and to separate information between different users. The more channel capacity required, the more channelization codes to be allocated. As for transmission power, dedicated channels employ closed loop transmit power control which provides more accurate power assignments resulting in less interference and lower bit error rate. Common channels typically employ open loop power control which is less accurate and not as well suited for transmitting large amounts of data.
Because of the bursty nature of packet data transmissions, congestion-sensitive transmission protocols, QoS parameters, and other dynamic factors of packet data transmissions, the channel-type best suited to efficiently support a user connection often changes during the life of the user connection. At one point, it might be better for the user connection to be supported by a dedicated channel, while at another point it might be better for the user connection to be supported by a common channel. A problem addressed by the present invention is determining if and when to make a channel-type switch during the course of a particular user connection.
One way of determining when to switch a user connection from a dedicated channel to a common channel is to monitor the amount of data currently being stored in a transmission buffer associated with that user connection. When the amount of data stored in the buffer is less than a certain threshold, that smaller amount of data may not justify the use of a dedicated channel. Thus, the connection may be switched to a common channel. On the other hand, the decrease in the amount of data to be transmitted for that user may only be temporary, given the dynamic aspects of data transmission, i.e., the amount of data in the buffer may quickly accumulate because of the load on the common channel or increased capacity needs for the connection. As a result, the connection may need to be switched right back to a dedicated channel.
Consider the situation where a user connection is currently assigned a dedicated radio channel having a higher data transmission rate/capacity than the current incoming rate of the user data to be transmitted over that channel. This situation might arise if there is congestion at some part of the Internet, e.g., Internet congestion causes TCP to dramatically reduce its transmission rate as described above. A slower incoming rate may also be the result of a xe2x80x9cweak linkxe2x80x9d in the connection external to the radio network, e.g., a low speed modem. In such situations, the radio transmit buffer is emptied faster than the data to be transmitted arrives. As a result of the slow incoming data rate, which may only be temporary, the user connection may be switched from the dedicated channel to a common channel, even though soon thereafter the user will have a large amount of data to transmit. Shortly after the user connection is switched to the common channel, the buffer fills up rapidly due to lower throughput on the common channel, and the user connection is switched back to a dedicated channel. These conditions may ultimately result in rapid, prolonged or cyclical switching back and forth between a common channel and a dedicated channel as long as such conditions persist. Such back-and-forth effects are undesirable because each channel type switch consumes power of the battery-operated terminal, loses packets during the switch, and requires additional control signaling overhead. Such back-and-forth switching is especially undesirable in environments where cell load (i.e., the amount of traffic in a particular cell) is low and channel resources are not in high demand.
FIGS. 1-2 illustrate a scenario where, for a given user, undesirable switching back-and-forth between dedicated and common channels is realized. FIG. 1 is a graph simulating a constant 32 kbit/sec incoming data stream to the transmission buffer where the user connection is assigned a dedicated channel with a capacity of 64 kbit/sec. The common channel capacity was simulated at 16 kbit/sec but is illustrated as 0 kbit/sec in FIG. 1. The buffer""s channel switch threshold which triggers a switch from dedicated-to-common channel and from common-to-dedicated channel is set at 1,000 bytes (i.e., when it is determined that less than 1,000 bytes are being stored in the buffer, this threshold triggers initiation of a timer whose expiration results in a switch from the dedicated channel to a common channel). An expiration timer may be set, e.g., to one second. FIG. 1 shows the allocated achieved channel capacity (in kbit/sec) plotted against time under these simulated conditions where the user connection is cyclically switched back and forth between a 64 kbps dedicated channel (after about one second) and a common channel (after less than 0.5 seconds).
FIG. 2 shows the buffer amount (in bytes) versus time for this same simulation. The buffer amount is approximately 600 bytes when the user is on the dedicated channel, which is below the threshold of 1,000 bytes. Therefore, the timer runs and upon its expiration the user connection is switched to the common channel. When on the common channel, the transmit buffer is filled very quickly by the 32 kbit/sec incoming stream (the incoming stream comes in at a rate faster than the rate at which data is output on the common channel) up to about 2000 bytes which, because it exceeds the 1000 byte threshold, results in a rapid channel switch back to the dedicated channel. This kind of rapid channel switch cycling is undesirable, as described earlier, because of the resources necessary to orchestrate each channel-type switch and the time required to set up a dedicated channel. Moreover, because traffic on the communications network may change over time, such cyclical switching may be more undesirable in low load conditions than in high load conditions when demand for dedicated channels is high. It is also undesireable for available dedicated or other high throughput channels to be left in a non-used state when they are available.
There may exist points in time when certain areas of the network may have light traffic thereon, while other areas of the network have heavy traffic thereon. In such situations, monitoring of total network traffic/load does not accurately reflect true network conditions. For purposes of example, in a cellular communications network, radio transmissions of each base station (BS) cover a geographical area known as a xe2x80x9ccell.xe2x80x9d Knowing the total load of the entire network does not translate into knowledge of load on a per cell basis. Therefore, many users in a low load cell may be allocated common channels when in reality allocation to them of dedicated channels would not place any undue burden on the network due to the light load in that cell.
In view of the above, it will be apparent to those skilled in the art that there exists a need in the art for a system and corresponding method which enables channel-type switching which takes network conditions and/or cell load(s) into consideration thereby reducing occurrence of any or all of the aforesaid problems in, e.g., low cell load environments. There also exists a need in the art to a system which enables high throughput (e.g., dedicated) channels to be used when they are available.
Timer timeout values (i.e., timer lengths) and/or buffer thresholds, used in determining if and when to switch from one type of channel to another for a given user, are chosen and/or dynamically adjusted based upon at least the traffic load in a cell in which the user is or has been located.
Data to be transmitted on a channel is stored in a transmit buffer. When the user has a first type of channel, a timer is started when less than a predetermined threshold amount of information is stored in the buffer. If the amount of information stored in the buffer does not exceed or pass above the threshold prior to expiration (or timeout) of the timer, then the user is switched from the first type of channel to a second type of channel. Conversely, when a user has the second type of channel, the user may be switched from the second type of channel to the first type of channel if the amount of information stored in the buffer exceeds a particular threshold. Timeout values of any or all such timers and/or any of these thresholds may be initially set or dynamically adjusted based at least in part upon a measured or estimated parameter such as the amount of cell traffic (i.e., load) in a cell(s) in which the user is (or has been) located. Other parameters and/or conditions may also be taken into account.
In an exemplary embodiment, the present invention may be implemented in a radio network control node having plural buffers, each buffer being assignable to support a mobile user connection and having a corresponding threshold. Channel-type switching circuitry, coupled to the buffers and/or corresponding timers, controllably switches a user connection from a first type of radio channel to a second type of radio channel. A calculator determines timeout values for timer(s). The thresholds and/or timeout values may be determined and/or dynamically adjusted during network operation based at least in part upon estimated or measured load in a cell(s) in which a user of the connection is or has been located. Upon receiving indication of expiration of a timer and/or passing of a threshold, a channel-type switching controller controls the channel-type switching circuitry to direct data corresponding to the mobile user connection stored at one of the buffers from a first type of radio channel (e.g., dedicated channel) currently supporting the mobile user connection to a second type of radio channel (e.g., common channel).
By taking into account the amount of traffic in at least a cell(s) of the user in determining thresholds and/or timer timeout values, actual network conditions are taken into account. For example, high throughput channels such as dedicated channels may be utilized when available. Moreover, unnecessary switching may be avoided or reduced when the amount of traffic in the cell(s) is low and plenty of channel resources are available. In other words, when there is little traffic in a particular cell, it may be beneficial to allow many or all users in that cell to use respective dedicated channels since there is no excess demand for the same. However, when there is substantial traffic in the cell, thresholds and/or timeout values may be adjusted to restrict dedicated channels to those users truly in need of them. Thus, rapid back-and-forth switching of a user from one type of channel to another may be avoided when not necessary. As an example, for increasing cell loads timer length(s) may be decreased in the context of determining when to switch from a dedicated channel to a common channel. Conversely, for decreasing cell loads timer length(s) may be increased in the context of determining when to switch from a dedicated channel to a common channel. The opposite may be true in the case of timer values utilized in determining when to switch from a common channel to a dedicated channel. This enables the system to conserve resources and overhead, and better reflect actual network conditions.